Category physics

Basics: Making graphs with kinematics stuff

rhettallain 4 Comments

**pre reqs:** [kinematics](http://blog.dotphys.net/2008/09/basics-kinematics/)

Suppose there is some experiment in which you throw a ball up and collect position and time data (with video analysis). What do you do with this data? Your instructor told you to make a graph, but how do you do that?

Here is the fictional data you (or I) collected:

![data2](http://blog.dotphys.net/wp-content/uploads/2008/09/data2.jpg)

Here is the text file with the data if you want to reproduce the graphs I make here [kinematics data](http://blog.dotphys.net/kinematics_data.txt)

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Computational Physics and a group of 1000 8th graders

rhettallain 1 Comment

I like computers, really I do. Computational physics is a good thing. However, there is a small problem. The problem is that there seems to be a large number of people out there that treat numerical methods and simulations as something different than theoretical calculations. You can tell who these people are because they refer to simulations as “experiments”. But what do these simulations really do in science? What is science really all about?

**Science**

To me, science is all about models. Making models, testing models, upgrading models. Models. Some examples are the model of gravity. One such model is that there is a gravitational force between any two objects with mass. This force is inversely proportional the square of the distance between them. (This is Newton’s model). Is this model perfect? No. Is this model the truth? No. How did this model come about? Experimental evidence.

**Models**

Well, how do you make models and what form can they take? To make a model, you collect some observations. The model should agree with these observations. This model could be a physical model (like the globe). It could be a mathematical model (like V=IR). It could be a numerical model – like a [vpython](http://vpython.org) program of a baseball trajectory with air resistance. These are all models.

**8th graders**

What does any of this have to do with 8th graders? I claim that any numerical calculation or simulation could be done with a group of 1000 8th graders rather than a computer. What does a computer do? (a computer program really) A program takes a problem and breaks it into a bunch a really small steps. It then does each of these steps and combines them together in some way. Just like a group of 8th graders with TI-89 calculators. Clearly, they are just computing something – they are not a separate type of science (other than theory and experiment).

Basics: Vectors and Vector Addition

rhettallain 7 Comments

**pre-reqs:** trig

Think of the following two things. Temperature and wind speed. These are two different things that you could measure, but there is one big difference. Wind speed has two parts to it – how fast and which direction. Temperature is just one thing (no direction). Temperature is an example of a scalar quantity (just one piece of information). Wind speed is an example of a vector quantity – multiple pieces of information. Here are some other examples:

**Scalar:** mass, money, density, volume, resistance
**Vector:** velocity (most physicist reserve the word “speed” to mean just the magnitude), acceleration, force, momentum, displacement, electric field

Ok, I get it – but who cares? Well, if you are taking an introductory physics course, you should care. Here is a question I like to ask to start the discussion of vectors:

If I move 3 feet and then 2 feet, how far am I from where I started?

The answer is that there is no answer. I commonly get the quick answer of 5 feet, although this is only one possible answer. Let me illustrate this question with some pictures.

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Amazing Blob Jump Launch Video Analysis

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Can you believe it? Have you seen this video?

Are you thinking what I am thinking? WOW. How could these people not follow my rules for cool internet video. Once again, here they are:
1 Keep the camera stationary. This way I don’t have to keep moving the origin in the movie.
2 Don’t Zoom. Same reason, this video followed that rule.
3 Include a clear and obvious calibration object. A meter stick would work, or even a Kobe Bryant (I can look up his height). Maybe it could be a Ford F-150 that has a known length. Something!
4 Include the mass and height of all people involved.
5 Use high quality video.
6 Don’t talk about fight club – oh wait, wrong list.

Despite failure to follow all these rules, I have managed to analyze this video. Really when I saw it, I said “wow” – was that real? It looked real, but who would get shot up that high? (it is on break.com, so fake is a possibility).

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Physics of Linerider IV: Friction?

rhettallain 1 Comment

Friction in Line Rider

Is there friction in Line Rider? Does it function as physics would expect? To test this, I set up a simple track:

![Page 6 1](http://blog.dotphys.net/wp-content/uploads/2008/09/page-6-1.jpg)

Basically, a slope with a flat part to start with and to end with. Let me show you something simple before further analysis:

![Page 6 2](http://blog.dotphys.net/wp-content/uploads/2008/09/page-6-2.jpg)

This is the x-position vs. time for the line rider on the first horizontal portion of the track (before he or she goes down the incline). This shows the rider traveling at a constant speed of 0.71 m/s. If friction were present, the rider would slow down. If you do not believe me (and why should you?) try creating your own line rider track with a long horizontal section. The rider will not stop, but continue on at a constant speed.

Ok, so no friction on the horizontal line. This makes a little bit of gaming sense. Who would want a rider to stop in the middle of the track and be stuck? That wouldn’t be fun. But, is there friction on non-horizontal portions? To test this, I will use the work-energy principle.

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Physics of Linerider III: Air Resistance

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There is no air resistance in line rider. Sorry to spoil the suspense.

To test for the presence of an air resistance force, a track was created that let the rider fall.

![linerider air 1](http://blog.dotphys.net/wp-content/uploads/2008/09/linerider-air-1.jpg)

(note the markers on the side. These are used to keep track of how the origin is moving).

Below is the y position of the rider as a function of time:

![linerider falling](http://blog.dotphys.net/wp-content/uploads/2008/09/linerider-falling.jpg)

In this situation, the rider falls about 100 meters. A quadratic line is fit to the data and an acceleration is obtained that is very similar to the previous case (where air resistance was assumed to be negligible). If there had been air resistance, this graph would have become more linear as the rider fell. Perhaps 100 meters is not far enough to fall, but in real life this should be far enough to detect the presence of an air resistance force. Or does it? Lets make a simple check.

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Physics of Linerider Part II: Scale

rhettallain 1 Comment

Scale of the Line Rider

First, we assume that the line rider is on Earth and for low speeds will have a free-falling acceleration of 9.8 m/s2. Next, an arbitrary distance is selected. In this case the length of the sled is chosen to be 1 LU (Linerider Unit).

![line rider](http://blog.dotphys.net/wp-content/uploads/2008/09/line-rider.jpg)

The goal will be to put the linerider in a free fall (where air resistance should be able to be ignored) and determine his (it could be a she, it is difficult to tell) acceleration in LU/s2. Then we can determine the conversion factor from LU/s2 to m/s2.

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The Physics of Linerider

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**Note: this is post from which IS my site, but I am putting here. – end note**
**Part I: Introduction**

[Linerider](http://linerider.com) is a flash “game” in which the user can create tracks. A rider is then allowed to slide down these tracks. If you have not played this, I recommend you DON’T play it. It is very addicting and can consume many hours of your time (hours you would otherwise spend on Digg or surfing needlessly).

In this short report, I will analyze the physics involved in line rider. An obvious question is “why not just ask the line rider programmer?”. Well, that would not be too much fun. Would it? So, there is the first reason – its fun. The second reason is to give an example of a physics project that students could do as a project.

I also tried to write this in a method that would be instructive. The analysis may seem overly detailed, but I tried to give a good background to the physics needed. If you had physics in college or high school, this could be a good review.

**Disclaimer** There are likely to be some mistakes in here. I know some people may get all uptight about some of my equations. I tried to simplify things as much as possible (only using vector notation when absolutely necessary). This could lead to some equations that experts might call wrong (but they are not wrong).

**UPDATE:** I would just like to emphasize that the goal is this report is NOT to say that the Line Rider game is bad. It is very, very good and very entertaining. I have also talked to a famous flash programer (who pretends he has a master’s degree in physics, but he doesn’t). This programmer explained that with flash, it is nearly impossible to correctly calculate things in flash and that most (if not all) of the physics is faked. So there.

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Significant figures what are they for and what do they have to do with uncertainty?

rhettallain 2 Comments

Suppose I am working on a problem and I wish to calculate the density of something. I measure the mass to be *m* = 24.5 grams and the volume is *V* = 10 cm3. In this case the density would be:
![Sigfig 1](http://blog.dotphys.net/wp-content/uploads/2008/09/sigfig-1.jpg)

ALERT! ALERT! ALERT! This is not a test!!!! Something is drastically wrong! Clearly I messed up. How can I have the mass measured to **3** significant figures, the volume measured to **1** significant figure, but the density calculated to **3** significant figures? Isn’t this a violation of some fundamental principle that could be worse than the Large Hardron Collider coming online and destroying the world?

No, we can all calm down. This really isn’t a big deal. Unfortunately many people (*hint* like chemists) do get all freaky about significant figures. Now calm down chemists. I am not saying significant figures are entirely stupid. They do have a purpose. What I AM saying is that they are not some fundamental rule that can not be broken.

So what is the deal with significant figures?

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MythBusters pulling on a phone book: You are doing it wrong.

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The MythBusters aren’t really doing it wrong, but they give me a chance to talk about some physics. In the latest show, they tested the myth that two phone books with their pages alternating were indestructible. To test this, they put the two phone books together and then pulled them apart in a sort of tug of war. Here is a diagram:

![tug1](http://blog.dotphys.net/wp-content/uploads/2008/09/tug1.jpg)

Looks great, what is wrong with this? The problem is that by pulling this way, the MythBusters produces 320 pounds of force on the book – but they could have done twice that. This really goes back to the old question: Which would produce a greater tension, two horses pulling in opposite directions, or one horse pulling on a rope tied to a tree. The answer is that both tensions are the same. However, many say that the two horses create a greater tension. The likely thinking in this “two horses are more” answer is that TWO things are doing something must be greater than ONE thing doing something. This reasoning fails because if you tie a rope to a tree, it is doing exactly the same thing the other horse doing: not moving.

Why? A force explanation follows:

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